1. Field of the Invention
This invention relates to lubricating oil compositions suitable for use in internal combustion engines.
2. Background
Contemporary lubricants such as engine oils use mixtures of additives such as dispersants, detergents, inhibitors, viscosity index improvers and the like to provide engine cleanliness and durability under a wide range of performance conditions of temperature, pressure, and lubricant service life.
It is critical to maintain sufficiently high viscosity at high operating temperatures to maintain a minimum lubricant film to minimize component wear. It is also critical to maintain a low, low-temperature viscosity to prevent excessive low-temperature oil thickening and to provide for satisfactory low-temperature operation. Viscosity index improvement can be a measure of such high- and low-temperature performance.
A variety of polymeric viscosity index improving components are used in various lubricating fluids to provide the necessary cross-grading to maintain fluid durability at high temperatures and to provide low viscosity at low temperatures to enhance low-temperature starting and low-temperature operation engine operation. These materials include high molecular weight hydrocarbons, polyesters and viscosity index improver dispersants that function as both a viscosity index improver and a dispersant. In particular, compositions such as styrene-diene copolymers, polymethacrylates, radial isoprene polymers, mixed olefin copolymers such as those chosen from the group consisting of ethylene-propylene copolymers and functionalized derivatives thereof are known. Many of the polymeric components used in the past have had deficiencies associated with the chemistry of the polymers such as shear instability and cleanliness properties. Additionally, the response of some of these added components are not as desirable as required for critical high performance considerations. Thus, there is a need for improved viscosity index improving materials.